Data tracks are concentrically formed on a hard disk. Data is read from or written on a magnetic disk after a seek operation. A head is moved along the radial direction on a rotating magnetic disk and positioned on a desired data track. To position the magnetic head on the desired data track, previously recorded head position identification codes and burst patterns are read by the head as described hereinafter.
The data area and the servo area are formed separately from each other. For example, as schematically shown in FIG. 1, data area 52 and servo area 50 are formed on disk 18. A magnetic head identifies a position on the disk in accordance with the servo data recorded on servo area 50 to write or read data on or from a desired data track.
The information of a cylinder (CYL) and a sector (SEC) is written on a disk as a gray code. Following the gray code, a burst pattern for providing a fine adjustment signal relative to each track is recorded on a disk. By reading this servo data, a magnetic head is positioned on a desired track.
FIG. 2 shows some of the head position identification code and burst patterns recorded on a magnetic disk. In FIG. 2, the magnetic disk rotates along the circumferential direction (direction of the arrow F in FIG. 2) and a magnetic head (not illustrated) moves along the radial direction of the magnetic disk (direction of the arrow G in FIG. 2). A plurality of data tracks 100A, 100B, 100C, . . . on which data is recorded are concentrically formed on the magnetic disk. The data tracks are arranged along the circumferential direction and an identification code recording area 102 and a burst-pattern recording area 104 are formed between adjacent data tracks.
Each data track is previously provided with a track address for identifying itself. The identification code comprises a predetermined number of bits. In these bits, a track address is recorded in the form of a gray code (Cyclic binary code). These bits are recorded in the identification code recording area 102. Moreover, there are a plurality of burst pattern strings (four strings in the case of FIG. 2) 106A, 106B, 106C, and 106D, in which each signal-recorded area (hatched portion in FIG. 2) is arranged along the radial direction of the disk. This burst pattern is recorded in the burst-pattern recording area 104.
To position the magnetic head on a desired data track, the current data track address is calculated in accordance with a read identification code each time the head faces the identification code recording area 102. The one-bit data recording length (L in FIG. 3(A)) in the identification code is previously determined as shown in FIG. 3(A). Therefore, portions to be magnetized to N or S in the recording range L are determined by the bit value "0" or "1" of the gray code representing the track address.
For example, when the magnetic head reads the identification code of data track N shown in FIG. 3(A), a pulse is generated at a portion magnetized to N or S as shown in FIG. 3(B). Moreover, when the magnetic head passes through the identification code recording area of the data track N+l shown in FIG. 3(A), a pulse is generated at a portion magnetized to N or S as shown in FIG. 3(C). It is possible to identify the value of a gray code recorded in the identification code recording area in accordance with the pulses read. The track address is determined by converting the read gray code to a binary code.
A gray code (100) is identified from the pulse string in FIG. 3(B) and a binary code (111) is obtained by converting the gray code (100). A gray code (000) is identified from the pulse string in FIG. 3(C) and a binary code (000) is obtained by further converting the gray code (000).
When the magnetic head reaches a desired data track, the head reads a plurality of signals obtained from the burst pattern strings in the burst-pattern recording area 104. This burst pattern generates a positioning signal whose level linearly changes in accordance with the position of the magnetic head. The magnetic head is positioned in accordance with the burst pattern signal so that the center of the gap of the magnetic head is located at the desired portion in the desired data track.
In a disk drive apparatus having a plurality of heads, heads are successively switched in a sequential data writing or reading operation. For example, in a disk drive apparatus having two disks and four magnetic heads with head numbers of 0, 1, 2, and 3, the heads are switched in the order of head numbers 0, 1, 2, and 3, and data is written or read. Conventionally, when a data write operation by the magnetic head 0 is terminated on a track of cylinder number 2500, the magnetic head 0 is switched to the magnetic head 1 and data is written at the cylinder 2500 of a disk facing the head 1. Similarly, data is written in the cylinder 2500 by the heads 2 and 3 in order and then, data is written in a cylinder 2501 by magnetic heads 0, 1, 2, and 3.
In a disk drive apparatus having a plurality of heads, each head is conventionally positioned on the corresponding position of the same cylinder number when magnetic heads are switched. In general, servo information for positioning a head is written on each disk surface when the disk drive apparatus is manufactured. As a result, the seek operation for moving a head to a target track is basically unnecessary under an ideal state when heads are changed. As shown in FIG. 4, this is the case in which a head is positioned on a predetermined track and other head is also located on the position of a track at the same cylinder position of an opposing disk. In FIGS. 4 and 5, each head is shown by shifting it in the circumferential direction of a track for ease of understanding. In fact, however, each head is not shifted in the circumferential direction of a track on a disk.
However, when setting a disk on which the servo information is written to a disk drive apparatus, a relative deviation between heads can be caused. This deviation is caused by following reasons. A relative deviation in each head for a deviation of the head's setting position. A gradient of the rotary shaft of the actuator for driving a magnetic head. A gradient of the spindle motor for driving a disk. Deformation of the actuator or spindle motor caused by a clamping force. When servo data is written by a servo writer, a servo pattern is written without any deviation between disks. Thereafter, however, a deformation occurs in a clamp removing process, such as removing the disk drive apparatus from the servo writer. Further another deformation occurs in the process for setting a top cover to the disk drive apparatus. Therefore, magnetic heads may deviate with respect to each other, and be positioned with respect to the disk drive apparatus as shown in FIG. 5. If a head switch operation is performed under the above state, Head 1 must move from Track n+1 on which Head 1 is located to Track n, that is, a seek operation across one track is required for sequential data writing or reading when switching the active head from Head 0 to Head 1.
As the tilt increases, that is, as the distance to the target track increases, the seek time increases. If a long seek operation is required in each head switch operation, the performance in a data write or read operation becomes worse.